The distribution of apoE between the major lipoprotein classes of normocholesterolemic plasma has been determined by molecular sieve chromatography, immunoaffinity chromatography, preparative ultracentrifugation, and polyanionic precipitation. Highly comparable values were obtained for the first two procedures (correlation coefficient (r) = 0.958), while the other procedures gave recoveries of apoE in high density lipoprotein that were respectively higher and lower, although total recoveries of apoE were essentially complete in each case. Immunoaffinity chromatography under the conditions described was not accompanied by detectable dissociation or redistribution of apoE. By immunoaffinity chromatography, all the apoE of native plasma was present in the form of complexes also containing either apoA-I or apoB. However, both ultracentrifugation and polyanionic precipitation methods of lipoprotein fractionation dissociated substantial proportions of apoE into both lipid-rich and lipid-poor forms that were unassociated with other apolipoproteins. These forms were derived mainly or exclusively from the dissociation of apoE from lipoproteins containing apoB, while apoE bound to apoA-I was dissociated by neither procedure. When lipoproteins were adsorbed on immobilized antibodies to apoA-I or apoB and dissociated with 3 M NaCNS, the apoE and apoA-I remained associated while the complex of apoB and apoE was substantially dissociated. These results suggest that immunoaffinity chromatography accurately determines apoE distribution in plasma. The results on the in vitro generation of unassociated apoE (Lp-E) are discussed in terms of the "family" concept of lipoprotein structure.